Bay Area Hiker: Geology
Rocks
Rock display at Sibley
Rocks on display at Sibley Volcanic Regional Preserve

Orinda Formation
Orinda Formation river gravels, sands, and mudstones, Volcanic Trail, Sibley Volcanic Regional Preserve

Rock formation at Round Valley
Miwok Trail, Round Valley Regional Preserve

Tafoni at Castle Rock
Tafoni, Castle Rock State Park

Boulder on trail at Mills Canyon
Mills Canyon Trail, Mills Canyon Nature Area

Rock on Fire Interpretive Trail
Fire Interpretive Trail, Mount Diablo State Park

Serpentine rock
Coastal Trail, Point Reyes National Seashore

Rock on Fire Interpretive Trail
Fire Interpretive Trail, Mount Diablo State Park

Graywackle Greywackle, Fire Interpretive Trail, Mount Diablo State Park

Seashell embedded in rock
Seashell embedded in rock, Upper Trail, Las Trampas Regional Wilderness

Rock at Marin Headlands
Wolf Ridge Trail, Marin Headlands, Golden Gate National Recreation Area

The distinctive greenblue serpentine color
Serpentine, Rock Springs Trail, Mount Tamalpais State Park

Granite with monkeyflower
Granite, North Peak Access Trail, McNee Ranch State Park

Rock on North Peak Access Road
North Peak Access Road, McNee Ranch State Park

Greenstone
Greenstone, Fire Interpretive Trail, Mount Diablo State Park

Chert Chert, Fire Interpretive Trail, Mount Diablo State Park

Tafoni
Tafoni, El Corte de Madera Open Space Preserve
Suggested reading: Geologic Trips: San Francisco and the Bay Area, by Ted Konigsmark (order this book from Amazon.com).